1. Field of the Invention
The present invention relates to an optical pickup device that irradiates an optical disc with an optical beam to achieve the reading and writing of information, and more particularly to the configuration of an optical pickup device that is ready for a multilayer optical disc. The invention also relates to an optical disc apparatus provided with such an optical pickup device.
2. Description of Related Art
Today, compact discs (hereinafter CDs) and digital versatile discs (hereinafter DVDs) are popular. In addition, in recent years, with the aim to increase the information capacity of optical discs, much research has been done on high-density recording on optical discs. For example, as high-definition DVDs, optical discs that permit the recording of vast amounts of information, namely HD-DVDs and Blu-ray discs (hereinafter BDs), have been put into practical use. Also, with the aim to achieve high-density recording on optical discs, multilayer optical discs, having a plurality of recording layers formed in the direction of their thickness, have been vigorously developed.
In such optical discs, a transparent cover layer is formed on a recording layer to protect it. Naturally, multilayer optical discs have such transparent cover layers and, here, the thickness of these transparent cover layers varies with the position of the recording layers they protect (here, a transparent layer formed between recording layers are also counted as a transparent cover layer). Thus, during playback from different recording layers of a multilayer optical disc, spherical aberration may arise and matter.
In particular, in cases where information is, for example, read from an optical disc with a high information recording density, such as a BD, by use of an optical pickup device, since the optical system of the optical pickup device includes an objective lens with a large numerical aperture (NA), the effect of spherical aberration ascribable to a difference in the thickness of transparent cover layers is magnified. For this reason, in particular, in an optical pickup device ready for a multilayer optical disc with a high information recording density, such as a BD, it is essential to provide a mechanism for correcting spherical aberration.
As to methods for correcting the spherical aberration occurring in the optical system of an optical pickup device, many reports have conventionally been made (see, for example, JP-A-2004-213784 and JP-A-2006-344274). Specifically, for example, a liquid crystal element having concentrically patterned electrodes is used, and the voltages applied to these electrodes are adjusted so as to correct spherical aberration; for another example, an expander lens composed of two lens elements is used, and at least one of those lens elements is driven so as to correct spherical aberration.
For yet another example, according to a method used these days, a collimate lens provided in an optical system is moved so as to change the optical beam incident on an objective lens between incidence from infinity and incidence from non-infinity; in this way, the state of the optical beam incident on the objective lens is changed so as to correct spherical aberration. This method relying on the driving of a collimate lens to correct spherical aberration requires less lens elements than the above-mentioned method employing an expander lens, and is therefore advantageous in miniaturization and cost reduction of optical pickup devices.
Thus, in cases where information is read from or written to a multilayer optical disc by use of an optical pickup device, even when spherical aberration arises and matters, if the optical pickup device is provided with a spherical aberration correction mechanism employing one of the above-mentioned methods, it is possible to correct spherical aberration and thereby prevent degradation in the quality of reading and writing information.
Inconveniently, however, the above-described configuration employing an expander lens or collimate lens and relying on the movement of the lens to correct spherical aberration has the following drawbacks.
In an optical pickup device, the optical beam emitted from a light source is condensed by an objective lens to form a condensed light spot on an optical disc. The size, or the diameter, of this condensed light spot generally depends on the numerical aperture (NA) of the objective lens, the wavelength (λ) of the light source, and the RIM intensity.
Here, the RIM intensity is an index representing the uniformity of the intensity distribution of the optical beam incident on the objective lens, and is defined as the ratio of the intensity of the part of the optical beam incident at the outermost edge of the objective lens to the intensity of the part of the optical beam incident at the center of the objective lens. That is, the RIM intensity is given by (the intensity of the part of the optical beam incident at the outermost edge of the objective lens)/(the intensity of the part of the optical beam incident at the center of the objective lens).
Of the three factors on which the diameter of the condensed light spot depends, the numerical aperture of the objective lens and the wavelength of the light source are largely determined by standards. Thus, to obtain satisfactory spot quality in the light beam condensed on a recording surface of an optical recording medium, it is important to obtain a satisfactory RIM intensity.
Inconveniently, however, in a configuration where an expander lens is provided, or a collimate lens is driven, to correct spherical aberration, moving the lens to correct spherical aberration causes the RIM intensity to vary, possibly making it impossible to obtain the desired RIM intensity. If the RIM intensity is lower than is desired, the diameter of the condensed light spot on the optical disc is larger than is desired, possibly degrading the quality of, for example, reading information.
On the other hand, in multilayer optical discs such as BDs, the number of layers is not limited to two and is expected to increase to four, six, and so forth. With this increase, the stroke over which the expander lens or collimate lens needs to be moved increases, and this is expected to aggravate the above-mentioned inconvenience of the variation of the RIM intensity.